1. Field of the Invention
The present invention generally relates to an optical recording medium, a memory apparatus, and a recording/reproduction method, more particularly, a high density optical recording medium, a memory apparatus, and a recording/reproduction method that use optical near field.
2. Description of the Related Art
In a conventional optical head for an optical disk, light is condensed by using a lens. In this case, the amount of information that can be recorded onto the optical disk depends on the diameter of the condensed beam. The diameter of such condensed beam is confined to the wavelength of the laser beam for recording and/or reproducing information to/from the optical disk, and by the numerical aperture (NA) of the optical lens system. In order to obtain a high density optical disk, it is required to shorten the wavelength of the laser beam and to increase the numerical aperture of the optical lens. Nevertheless, due to limits in diffraction, there are limits in obtaining such high density optical disks with such methods.
In order to increase the numerical aperture of the optical lens system, a method for recording information onto an optical disk medium is proposed, in which a numerical aperture (NA) of 1 or more is obtained by employing a SIL (Solid Immersion Lens) and utilizing the evanescent light leaking from the bottom of the SIL. Nevertheless, recording information onto the optical disk medium by utilizing the evanescent light requires the distance between the SIL and the recording surface of the optical disk to be maintained at no more than 1/10 of the wavelength. Therefore, precise control is required for maintaining a constant distance between the optical disk and the SIL. There is also a problem where dust may adhere to the optical disk to cause contact with the SIL. The problem of dust causes difficulty in attaining removability, which is one of the benefits of the optical disk. Furthermore, there are limits in obtaining a high density optical disk by employing the SIL since the numerical aperture (NA) is increased based on the refractive index of the SIL.
Meanwhile, a method for recording and/or reproducing information to/from an optical disk is proposed, in which an optical near field is used for increasing capacity of an optical disk by intensifying the density of the optical disk.
As for a widely used optical near field probe serving to generate the optical near field, there is an optical fiber (optical fiber probe) having a sharpened tip with a fine opening of a size no more than the wavelength of the laser beam used for recording and/or recording. The optical fiber probe is manufactured by applying metal coating to portions other than its tip portion after heating and stretching an end of an optical fiber or after tapering an optical fiber with a chemical etching method. Accordingly, by directing an incident laser beam on the optical fiber probe, an optical near field can be generated at the proximity of the fine opening formed at the tip portion.
Nevertheless, the optical fiber probe has a drawback of inefficient utilization of light. For example, in a case where the aperture diameter is 100 nm, the ratio of the optical strength of the incident laser beam on the optical fiber probe to the laser beam emitted from the tip portion of the optical fiber probe becomes 0.001% or less, thereby resulting to an emitted laser beam with considerably low optical strength. In order to solve this problem, the below-given optical near field probes are proposed.
The first probe is a multi-level taper optical fiber probe. This optical fiber probe is described in “Applied Physics Letters” (Vol.68, No. 19, p. 2612-2614) issued in 1996, and “Applied Physics Letters” (Vol. 73, No. 15, p. 2090-2092) issued in 1998. This optical fiber probe has its pointed tip formed in two or three varying levels from its root portion to its tip portion.
The next probe is a metal needle probe. This probe is described in Japanese Laid-Open Patent Application No. 6-137847, in which light is irradiated to a tip of a needle for generating an optical near field in proximity to the tip portion.
The next probe is a fine aperture fiber probe with a fine metal sphere. This fiber probe is described in Japanese Laid-Open Patent Application No. 11-101809, in which a fine metal sphere is formed at a center of a fine aperture of a tip portion of the fiber probe. In this fiber probe, an optical near field is generated by irradiating light from the fine aperture and thereby exciting plasmon in the fine metal sphere.
The next probe is a probe with a metal coated glass segment. This probe is described in “Physical Review B” (Vol. 55, No. 12, p. 7977-7984) issued in 1997. This probe has a metal film with a thickness of approximately 50 nm formed on a triangularly cut-out glass segment, in which surface plasmon is excited on the metal film. Accordingly, since the excited surface plasmon is propagated toward the tip of the probe, a strong optical near field is generated in proximity to the tip.
The next probe is a glass substrate probe with a metal scatterer. This probe is described in Japanese Laid-Open Patent Application No. 11-250460. This is a probe having a metal scatterer attached to a bottom portion of a glass substrate. By attaching the metal scatterer, a strong optical near field is generated in proximity to the metal scatterer.
In near field optics, it is necessary to maintain a space of approximately a few nanometers (nm) to several tens of nanometers between an optical near field generating member and the recording surface. Accordingly, in the above-described probes formed with the optical fiber or the glass segment, a particular control system is necessary for precisely controlling the space between the tip portion of the probe and the recording surface. This control system, in general, measures the space between the tip portion of the probe and the recording surface by measuring the atomic force between the tip portion of the probe and the recording surface, and servo controls the position of the probe so that the measured value is constant. Nevertheless, since there is a certain limit to the servo control area, the relative scanning speed of the probe with respect to a recording medium is to be no more than a prescribed speed with an error no more than a prescribed value. Particularly, in an optical disk memory device required to provide high speed data transfer, it is necessary to increase the speed at which the probe can scan the optical disk. Nevertheless, in a case where a disk having tilt or skew is rotated at high speed, the servo control system may be unable to provide sufficient control since the probe is required to trace large disturbances with high frequency amplitude. In order to solve the problem, the below described probe is proposed.
The first probe is a flat aperture optic fiber probe. This optical fiber probe is described in “Electronics and Communications in Japan” (Part 2, Vol. 81, No. 8, p. 41-48) issued in 1998 (translated from “Journal of The Institute of Electronics, Information, and Communication Engineers Vol. J81-C-I, No. 3, p. 119-126). This probe has an aperture formed by applying an anisotropic etching technique to a silicon substrate. Since the portion surrounding the aperture is flat, the distance between the tip portion of the probe and the recording surface can be maintained constant when the probe approaches the recording medium.
The next probe is a pad aperture probe. This probe is described in Japanese Laid-Open Patent Application No. 11-265520. This probe has a quadrangular pyramid-shaped protrusion with a fine aperture formed at a tip portion thereof, and also has a pad formed at a surrounding portion of the protrusion. The pad serves to maintain the distance between the tip portion of the probe and the recording surface constant.
The next probe is a plane illumination laser probe with a fine metal chip. This probe is described in “Applied Physics” (Vol. 68, No. 12, p. 1380-1383) issued in 1999. This probe has a fine metal protrusion with a fine aperture formed at an end face of the plane illumination laser port. Since the probe has a flat structure, the distance between the tip portion of the probe and the recording surface can be maintained constant when the probe approaches the recording medium.
The next probe is described in “Optics Communications” (Vol. 69, Nos. 3 and 4, p. 219-224) issued in 1989. This probe is designed to efficiently generate a slight optical near field. This probe efficiently generates the optical near field by applying light to a patch antenna and a coaxial cable.
Another probe is described in U.S. Pat. No. 5,696,372 issued on Dec. 9, 1997 (Grober et al.). This probe is also aimed at efficiently generating a slight optical near field. This probe employs a fine dipole antenna formed with a bow-tie shaped metal segment for generating a slight optical near field.
With a system for recording and/or reproducing information by using an optical near field, the three below given factors are required to be satisfied. First, it is required to precisely control the distance between the optical near field probe or the like serving to generate the optical near field and the recording medium to be a constant distance shorter than the wavelength of the laser beam used for recording/reproducing information. Second, the beam spot of the generated optical near field is required to be of a fine size. Third, the generated optical near field is required to be of high light use efficiency.
The above-described multi-stage pointed optical fiber probe has a light use efficiency which is approximately 10 to 100 times higher than that of a typically used fiber probe, but is still insufficient to be applied for optical recording/reproduction, which requires a high light use efficiency of 10% or more. In addition, it is mechanically fragile, and is especially vulnerable in high speed scanning since optical fiber is used.
All of the metal needle probe, the fine aperture fiber probe with a fine metal sphere, the probe with a metal coated glass segment, and the glass substrate probe with a metal scatterer are able to attain high light use efficiency by utilizing their metal property. All of them, however, are formed with a mechanically fragile tip, and are, therefore, particularly vulnerable in high speed scanning. Particularly, the metal pin probe and the glass substrate probe with a metal scatterer have a problem in that a large amount of background light is detected therefrom since the light that is irradiated neither to the tip of the needle nor the scatterer becomes incident on the recording medium.
Furthermore, as described above, some other probes capable of providing high speed scanning are also proposed. Although the flat aperture probe and the pad aperture probe can be used in high speed scanning, their light use efficiency is low.
The plane illumination laser probe with a fine metal chip is expected to provide high speed scanning performance, high light use efficiency, and little background light. In generating a strong optical near field by using the metal fine protrusion of the probe, the form of the metal is to be optimized. Nevertheless, neither the form nor the method of manufacture is disclosed.
With the probe for efficiently generating an optical near field by applying light to a patch antenna and a coaxial cable, or with the probe employing a fine dipole antenna formed with a bow-tie shaped metal segment for generating a slight optical near field, there still is a problem with dust on the recording medium, and a problem in precisely controlling the distance between the optical near field generating member and the recording medium to a constant minute distance that is shorter than the wavelength of the laser beam used for recording/reproduction (head/disk interface problem). It is, therefore, difficult to attain removability, which is one of the advantages of an optical disk.
Meanwhile, a method where a lens-like substrate is disposed in an optical disk medium for improving recording density is disclosed in “Optical Data Storage 2001 Technical Digest of 2001, April 22-25, (p. 277-279, Guerra et al.). This method is aimed at solving the dust problem and the head/disk interface problem. However, this method does not use an optical near field, but uses light condensed by a micro-lens installed in a recording medium for recording/reproducing with the recording medium. This enables recording density to be increased by increasing the refractive index of the material of the lens. Nevertheless, as described in the document, the track pitch can merely be reduced to approximately one half when using a typical optical system since there are limits to the refractive index. As a result, its recording density can only be enhanced to approximately two times. Furthermore, manufacture of the recording medium is difficult since the recording medium requires a spherical lens-shaped substrate to be uniform at its periphery. Furthermore, the track pitch can only be narrowed to a small degree, and recording density cannot be largely increased since it is possible that the beam may diffuse to an adjacent track and cause problems such as cross-talk, cross-erase, and cross-write. In addition, the lens uses, for example, TiO2 as its material since a refractive index of 2.7 is required. With such material, however, light cannot pass through in a case of employing a blue-violet laser diode (LD) with a wavelength of 410 nm. Accordingly, the method cannot be applied to a short wavelength laser diode, and cannot attain high recording density.